(Cosmos) – Just 10 rivers – eight of them in Asia – may be responsible for dumping almost four million tonnes of plastic into the seas every year.

Calculating with precision the source and amount of plastic trash in the oceans is difficult; estimates tend to cover wide ranges. Previous research has found about one-fifth of total ocean plastic trash comes from marine activities – plastic tossed from fishing boats, ships, drilling platforms and so on. The four-fifths from land totals 4.8 million to 12.7 million tonnes a year.

While most plastic comes from coastal cities and towns, where plastic litter is washed into storm drains and flows into the sea, new research by scientists from the Helmholtz Centre for Environmental Research and the Weihenstephan-Triesdorf University of Applied Science, both in Germany, highlights the importance of better management of upstream systems to reduce the significant proportion of plastic transferred via rivers.

ABSTRACT: A substantial fraction of marine plastic debris originates from land-based sources and rivers potentially act as a major transport pathway for all sizes of plastic debris. We analyzed a global compilation of data on plastic debris in the water column across a wide range of river sizes. Plastic debris loads, both microplastic (particles <5 mm) and macroplastic (particles >5 mm) are positively related to the mismanaged plastic waste (MMPW) generated in the river catchments. This relationship is nonlinear where large rivers with population-rich catchments delivering a disproportionately higher fraction of MMPW into the sea. The 10 top-ranked rivers transport 88–95% of the global load into the sea. Using MMPW as a predictor we calculate the global plastic debris inputs form rivers into the sea to range between 0.41 and 4 × 106 t/y. Due to the limited amount of data high uncertainties were expected and ultimately confirmed. The empirical analysis to quantify plastic loads in rivers can be extended easily by additional potential predictors other than MMPW, for example, hydrological conditions.